There is an image processing apparatus capable of reading an irregular sheet such as a voucher or a receipt as an original document which is a reading object. In the reading of such an irregular sheet, first, a range including the whole sheet is read. A sheet part and a part of material (for example, a sheet cover coated on a sheet table) of a sheet back surface are contained in a read image. The irregular sheet is generally read by extracting an image of the sheet part from an image read in this manner.
However, if a color of a material of the sheet back surface is similar to a color of a ground of the sheet, there is a case in which it is difficult to identify a contour line on the sheet part and it is impossible to correctly extract the image of the sheet part. The sheet cover is generally constituted by white material which is hard to cover an image at the time of copying, and the background of the sheet which is the reading object is mostly white. Thus, in the image data acquired by reading, there is a case where it is difficult to distinguish the white part of the sheet cover and the white part of the sheet ground. Conventionally, although there are some proposals of an image processing apparatus including a component for solving such a problem, in each of those proposals, a member having a color distinguishable from the sheet part is arranged on the sheet back surface. In such a construction, a special mechanism which controls addition of the member and a position of the member is necessary, which may lead to an increase in manufacturing cost.